The present invention relates to exhaust gas sensors. More particularly, the present invention relates to an exhaust gas sensor having an lower protective shield and method for making the same.
Exhaust sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, oxygen sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, for example, to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:       E    =                  (                              -            RT                                4            ⁢            F                          )            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                                                              where            :                    ⁢                      xe2x80x83                                                        E          =                      xe2x80x83                    ⁢                      electromotive            ⁢                          xe2x80x83                        ⁢            force                                                        R          =                      xe2x80x83                    ⁢                      universal            ⁢                          xe2x80x83                        ⁢            gas            ⁢                          xe2x80x83                        ⁢            constant                                                        F          =                      xe2x80x83                    ⁢                      Faraday            ⁢                          xe2x80x83                        ⁢            constant                                                        T          =                      xe2x80x83                    ⁢                      absolute            ⁢                          xe2x80x83                        ⁢            temperature            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                        ref                    =                      xe2x80x83                    ⁢                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            reference            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                                =                      xe2x80x83                    ⁢                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            exhaust            ⁢                          xe2x80x83                        ⁢            gas                              
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture. Increased demand for improved fuel economy and emissions control has necessitated the development of oxygen sensors capable of quantifying the exhaust oxygen partial pressure over a wide range of air fuel mixtures in both fuel-rich and fuel-lean conditions.
Sensors are typically sealed from the outside environment to prevent water intrusion and environmental contaminants from entering the sensor. Sensors are also typically sealed from the exhaust using high temperature sealing materials (for example, talc or glass) designed to withstand spark-ignition environments (where the exhaust can reach temperatures up to about 1,000xc2x0 C.). Consequently, great care and time consuming effort must be taken to prevent the sensor""s sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc.
Particularly, an exhaust sensor requires a protective shield (lower shield) around the tip of the sensor element that allows exhaust gas to be sensed while simultaneously protecting the sensor element. Two types of protective shields are commonly used. The first type is a single shield generally having slits to allow for the passage of exhaust gas. Typically, this type of shield is used where precise control of temperature about the sensor element is desired. The second type is a dual shield having both internal and external shields.
A dual shielded protective shield 30 is shown in prior art FIG. 1. Internal shield 35 is disposed within and surrounded by outer shield 34. Within internal shield 35, sensing element lower end 82 is disposed for sensing exhaust gas. Internal shield 35 helps to isolate radiative heat that would affect the sensing element. Also, the internal shield 35 helps to prevent direct impingement from solid and liquid exhaust matter onto the sensing element. To vary the amount of exhaust gas contacting the sensing element, the size, number, and placement of apertures 38 and 39 can be varied. The apertures 38 and 39 are costly to manufacture. The holes are generally made by a punching technique where round disks are removed from a rectangular sheet. The remnant portions, i.e., the disks become waste thereby creating about 40 to about 50% scrap material. As the shields are comprised largely of temperature resistant materials such as nickel, the scrap cost is significant relative to the total cost of the lower shield. In addition to material costs, the tooling costs are significant because the apertures must be punched from the sides of the sheet metal shields without deforming the shield structure.
Accordingly, there remains a need in the art for a low cost, contaminant resistant protective shield arrangement.
The problems and disadvantages of the prior art are overcome and alleviated by the protective shield arrangement for an exhaust sensor and method of making and using the same. This is achieved by providing a protective shield for an exhaust gas sensor that comprises a porous mesh material formed as a cap for placement over an end portion of a sensing element which is made by a method that comprises providing a unitary piece of the mesh material, and folding the mesh material to form the cap.
Thereafter, the cap can be used as a protective shield by affixing the cap over an end portion of a sensing element of an exhaust sensor; supplying a gas to be sensed; and sensing the gas wherein the sensing comprises supplying the gas to an exterior of the protective shield, passing the gas through a tortuous pathway within the mesh material and into an interior portion of the protective shield, and exposing the sensing element to the gas.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.